In some forms of surveillance system, especially those operating by the detection of radiation in the infra-red spectrum, a multi-element array of detector elements is used to scan an image of the field of view. The different detectors scan different parts of the scene and, generally, each detector is provided with its own video amplifying channel and is coupled to its respective amplifier by means of a.c. coupling. However, since different detectors scan different parts of the scene, their output wave forms will in general be different and the a.c. coupling will progressively charge in each raster scan to result in different mean voltages. To produce an image of the scene with high fidelity it becomes necessary to make the a.c. coupling time constants long enough to prevent the differences in the voltages taken up by the different a.c. couplings from becoming large enough to become perceptible in the course of one raster scan. At the end of each raster scan it is then necessary to restore the charges taken up by the different couplings to a common value while the detectors are looking at a common reference source of radiation. Thus all the channels begin the next scan reset to a common level so that if the respective detectors scan across a uniform patch of the scene the image of the latter will appear uniform in the display.
This invention relates to scanning arrangements which are suitable for but not necessarily restricted to surveillance systems. Herein it is described in relation to surveillance scanners. It has as its object the provision of scanning apparatus in which the detector video amplifier channels have improved characteristics.
It is conventional to provide a reference source of radiation within the scanning apparatus. However, the radiation output of the reference source may sometimes differ greatly from the radiation received from the field of view and under these circumstances large differences between the detector outputs when viewing the reference source and when viewing the scene may be unavoidable. In such circumstances unless the gains of the different detector amplifiers are precisely the same, the display output which is set equal when viewing the reference source will nevertheless differ when viewing a uniform area of the scene.